Increasingly smaller geometries and feature sizes of semiconductor devices have made transmission electron microscopy (TEM) an increasing important characterization technique for semiconductor product development and failure analysis. Resolution of less than fifteen hundredths of a nanometer (0.15 nm) allows TEM to provide structural information down to the atomic level. TEM can also be used in conjunction with X-ray or electron energy loss spectrometers to provide information on chemical composition at sub-nanometer length scales. See, for example, D. B. Williams and C. B. Carter, Transmission Electron Microscopy. A Textbook for Materials Science, Plenum Press, New York, 1996.
The chief difficulty associated with TEM has traditionally been the preparation of samples. D. K. Schroeder, Semiconductor Material and Device Characterization, Second Edition, John Wiley & Sons, New York, 1998. Samples for TEM analysis must be less than two hundred fifty nanometers (250 nm) thick in order to achieve good image quality. The increasing demand for TEM analysis in the semiconductor industry has placed a high priority on reducing the time and labor required to prepare a TEM sample.
One of the earliest techniques for preparing TEM samples from semiconductor materials is mechanical lapping-polishing. S. J. Klepieis, J. P. Benedict and R. M. Anderson, Specimen Preparation for Transmission Electron Microscopy of Materials, Materials Research Society Symposium Proceedings 115, p. 179, Materials Research Society, Pittsburgh, Pa., 1988. In mechanical lapping-polishing a small piece of semiconductor wafer containing the sample site is mounted on a tripod polisher. The tripod polisher holds the wafer against a rotating abrasive disc. A series of progressively finer abrasives are used with water rinse to reduce the sample to electron transparency. The mechanical lapping-polishing technique typically takes four to five hours to complete. The mechanical lapping-polishing technique can be used to prepare both plan-view samples and cross-sectional samples of a semiconductor wafer. FIG. 1 illustrates a schematic view of one half of an exemplary prior art semiconductor wafer 100 showing an exemplary portion of a plan-view 110 of the wafer 100 and an exemplary portion of a cross-sectional view 120 of the wafer 100.
The prior art mechanical lapping-polishing technique can also be performed automatically. Sagitta Corporation has developed an automatic tool for mechanically lapping-polishing both TEM samples and Scanning Electron Microscopy (SEM) samples. G. A. Schechter, L. Adams, and I. Ward, SEM/TEM Sample Preparation, Solid State Technology, pp. S1–S8, September, 2000. The Sagitta automatic tool speeds up the polishing process by removing all of the manual inspections required to check the progress during the tripod polishing process in order to avoid polishing away the site of interest.
The past fifteen (15) years have seen a trend toward the increased use of focused ion beam (FIB) milling tools for the preparation of TEM samples. Site-specific TEM samples can be prepared faster and easier using an FIB milling tool than by polishing. FIB preparation is typically done with a dual beam instrument employing an ion column for milling and an SEM column for imaging. The ion column uses a beam of gallium ions to remove unwanted material and expose a fresh surface passing through the region of interest. Progress is monitored with secondary electron images taking throughout the milling process.
Several techniques have been developed to make FIB-milled cross-sectional samples compatible with the standard three millimeter (3 mm) diameter TEM sample holder. One such technique is to use tripod polishing for the initial sizing of the sample. R. Anderson and S. J. Klepeis, Specimen Preparation for Transmission Electron Microscopy of Material IV, Materials Research Society Symposium Proceedings V.480, p. 187, Materials Research Society, Pittsburgh, Pa., 1997. In this approach the sample is polished to a thickness of approximately thirty microns (30 μm) and then mounted on a modified copper support grid that is compatible with milling in the FIB and with the TEM sample holder. Prior to use, the sample holder grid is modified by manually cutting away one parallel side to allow access for the ion beam during the milling process. Finally, the sample is transferred into an FIB instrument for ultimate thinning to electron transparency. This approach can be used for both plan-view samples and cross-section samples.
Another approach for making FIB preparation compatible with TEM sample holders is a technique known as “μ-sampling”. This approach involves the transfer of FIB-milled TEM samples directly from a semiconductor chip or wafer onto a copper support grid using micro-manipulators. See, for example, M. H. F. Overwijk, F. C. van den Heuvel, and C. W. T. Bulle-Lieuwma, Journal of Vacuum Science and Technology, B11, p. 2021 (1993); L. A. Giannuzzi, J. L. Drown, S. R. Brown, R. B. Irwin, and F. A. Steive, Materials Research Society Symposium Proceedings, Volume 480 on Specimen Preparation for Transmission Electron Microscopy of Materials IV, p. 19, American Institute of Physics, New York, 1997; L. A. Giannuzzi, J. L. Drown, S. R. Brown, R. B. Irwin, and F. A. Steive, Micros. Res. Tech. 41, p. 285, 1998; and T. Onishi, H. Koike, T. Ishitani, S. Tomimatsu, K. Umemura, and T. Kamino, A New Focused-Ion Beam Microsampling Technique for TEM Observation of Site-Specific Areas, Proceedings of the Twenty Fifth International Symposium for Testing and Failure Analysis, pp. 449–453, 1999.
The “μ-sampling” technique avoids many of the “pre-preparation” steps such as wafer cleaving, sawing, and tripod polishing, but it does so at the expense of increased time spent on the FIB preparation of the sample. The high cost and heavy usage commonly associated with FIB instruments make any increase in instrument time for sample preparation a potential factor in bottlenecking laboratory output.
A third approach for making FIB preparation compatible with TEM sample holders is a technique developed by the Semiconductor Engineering Laboratories Corporation (SELA). The technique will be referred to as the “SELA process”. A more detailed discussion of the SELA process is set forth in W. D. Kaplan, R. Oviedo, K. Kisslinger, E. M. Raz and C. Smith, Automatic TEM Sample Preparation, Proceedings of the Twenty Fifth International Symposium for Testing and Failure Analysis, pp. 103–107, 1999; and in R. Reyes, F. Shaapur, D. Griffiths, A. C. Diebold, B. Foran, and E. Raz, Automated SEM and TEM Sample Preparation Applied to Copper/Low K Materials, AIP Conference Proceedings, pp. 580–585, 2001.
The SELA process automates the initial wafer cleaving and thinning steps for cross-sectional semiconductor sample preparation. The SELA process proceeds in two steps. In the first step an automated microcleaver is used to cut the samples. The automated microcleaver has an accuracy of one fourth of a micron (0.25 μm). In the second step the samples that are output from the automated microcleaver are then provided to a rotary diamond saw (referred to as a “TEMpro™” unit). An earlier version of the TEMpro™ unit is referred to as a “TEMstation™” unit. “TEMpro™” and “TEMstation™” are trademarks of the Semiconductor Engineering Laboratories Corporation (SELA).
The two SELA tools (i.e., the automated microcleaver and the rotary diamond saw (“TEMpro™”)) automatically mount the sample on a TEM-compatible copper support grid and use a series of diamond saws to reduce the sample to an approximate thickness of thirty microns (30 μm). The result is a TEM-compatible, site-specific, cross-section sample that is ready for FIB milling. By consistently and quickly achieving a sample thickness of thirty microns (30 μm) the SELA process is aimed at a high throughput of TEM samples by eliminating the steps of tripod polishing and by minimizing FIB instrument time.
The prior art SELA process provides automated sample preparation for obtaining cross-sectional views of transmission electron microscopy. However, there is no similar method for preparing samples for plan view transmission electron microscopy.
There is therefore a need in the art for a system and method for providing automated sample preparation for plan view transmission electron microscopy. There is also a need in the art for a system and method that automatically prepares plan view transmission electron microscopy samples in manner that is not labor intensive.